It is known that polyester fiber materials such as polyester fiber fabrics, polyester fiber cords and polyester fiber threads, have an excellent tensile strength and dimensional stability, and therefore, are useful as a reinforcing material for car tires, conveyor belts, V-belts and hoses. However, since the polyester fiber materials inherently have a poor bonding property to rubber, in order to utilize the polyester fiber material as a reinforcing material for rubber articles, it is required to significantly improve the bonding property of the polyester fiber material to rubber. For this reason, a number of approaches have been taken in attempts to improve the bonding property of the polyester fiber material to rubber. However, during the approaches, it was found that the improvement in the bonding property of the polyester fiber material to rubber causes the resultant improved polyester fiber material to have an excessively high stiffness, a poor processability in the shaping or molding process and a poor resistance to fatigue fracture. For example, U.S. Pat. No. 3,307,966 discloses a process for improving the bonding property of the polyester fiber material to rubber, by first impregnating the polyester fiber material with a first treating liquid containing a polyepoxide compound and an aromatic polyisocyanate compound, and then, by second impregnating the first impregnated polyester fiber material with a second treating liquid containing a reaction product of resorcin with formaldehyde and a rubber latex. The resultant product of the above-mentioned process exhibits a relatively superior bonding property to rubber. However, this known process causes the resultant product to have a relatively high stiffness and, therefore, it is difficult to bend the product during the shaping operation, and the product has a remarkably decreased resistance to fatigue fracture.
Japanese Patent Application Publication (Kokoku) No. 429004 discloses a process in which a polyester fiber material is treated with a first treating liquid containing an epoxy resin, a ethyleneimine compound and a rubber latex and, thereafter, the thus treated polyester fiber material is further treated with a second treating liquid containing a reaction product of resorcin with formaldehyde and a rubber latex. Also, U.S. Pat. No. 3,460,973 discloses a process in which a polyester fiber material is treated with a first treating liquid containing a lactam-blocked polyisocyanate compound, an emulsifying agent and a rubber latex, and then, with a second treating liquid containing a reaction product of resorcin with formaldehyde and a rubber latex. Furthermore, British Pat. No. 1,056,798 discloses a process in which a polyester fiber material is treated with a single treating liquid containing a blocked isocyanate compound, an epoxy resin and a rubber latex without using an additional treating liquid containing a resorcin-formaldehyde resin and a rubber latex. The above-mentioned three processes cause the resultant products to have a proper softness. However, these resultant products have a relatively low bonding property to rubber and, therefore, a poor rubber coverage. The term "rubber coverage" used herein refers to a percentage of total area of portions of the reinforcing material covered with rubber when the reinforcing material is peeled off from the rubber article in which the reinforcing material is embedded within a rubber matrix.
As is clear from the above description, the conventional processes all failed to satisfy all of the requirements of proper softness, high bonding property to rubber and high resistance to fatigue fracture of the rubber reinforcing polyester fiber materials.
Also, it is known that generally isocyanate compounds are easily decomposed when water comes into contact with the isocyanate compounds. Therefore, when isocyanate compounds are used, it is necessary to protect them from water. That is, in order to prepare a treating liquid containing isocyanate compounds, it is necessary to use an organic solvent free from water. This organic solvent must be recovered from the treating process in order to avoid environmental pollution. This necessity causes the treating process to be costly and the working environment for the treating process to be polluted.
Accordingly, in order to avoid the above-mentioned problem, various types of blocked isocyanate compounds which are stable in water, were used for treating the polyester fiber material.
Usually, conventional blocked isocyanate compounds have a high degree of crystallizing property. Therefore, the stability of aqueous dispersions of the blocked isocyanate compounds are poor, to an extent that it is necessary to continuously agitate the aqueous dispersion so as to keep the particles of the blocked isocyanate uniformly dispersed in water. Also, when the aqueous dispersion is used, it is difficult to apply the blocked isocyanate particles uniformly on the polyester fiber material. In order to obtain a satisfactorily uniform bonding property, it is necessary to apply a large amount of the aqueous dispersion of the blocked isocyanate to the polyester fiber material. Even if a large amount of the blocked isocyanate is used, the resultant product exhibits an unsatisfactory bonding property to rubber and rubber coverage.
Japanese Patent Application Publication (Kokoku) No. 47-49768(1972) and Japanese Patent Application Laid-open No. 54-112295(1979) disclose a process in which a polyester fiber material is impregnated with an impregnating liquid containing an epoxide compound in the presence or in the absence of an epoxy-hardening agent, the impregnated product is heat-treated at a temperature of 150.degree. to 260.degree. C., the heat-treated product is further impregnated with an impregnating liquid containing an ethylene urea compound, a blocked isocyanate compound, a resorcin-formaldehyde reaction product and a rubber latex, and finally, the further impregnated product is heat treated at a temperature of 150.degree. to 260.degree. C. However, in the above-mentioned process, it is difficult to prepare a stable impregnating liquid containing dispersions of the blocked isocyanate compound and the ethylene urea compound having a high degree of crystallizing property and a high degree of hydrophobic property. When the impregnating liquid is stored or allowed to stand, the stability of the impregnating liquid deteriorates with the lapse of time. Also, the ethylene urea compound is in the form of coarse particles having a relatively large size and, therefore, exhibits a poor contribution in the enhancement of the bonding property and the rubber coverage.
In recent years, the form of a V-belt has changed from a conventional wrapped V-belt in which cotton cord fabric is used, to a new raw edge type V-belt in which the polyester fiber material is used as a reinforcing material. In this type of V-belt, the polyester fiber reinforcing material is arranged in the side surface portions of the V-belt so as to expose it to the outside of the V-belt. That is, when the V-belt is used, the side surface of the belt made by the polyester fiber reinforcing material comes into direct contact with a pulley. This direct contact causes the exposed polyester fiber reinforcing material to be abraded and the durability of the V-belt to be degraded. Therefore, in a raw edge V-belt, it is necessary that the polyester fiber reinforcing material exhibits an enhanced resistance to abrasion and that the polyester fiber yarns in the reinforcing material are highly resistive to separation from the rubber matrix. Also, it is necessary that the rubber matrix in the raw edge V-belt has a high degree of resistance to abrasion. In order to attain the above-mentioned property, the polyester fiber reinforcing material must be firmly fixed to the rubber matrix and must exhibit a proper stiffness which is effective for enhancing the resistance of the V-belt to abrasion.
For the above-mentioned purpose, it was attempted to produce a polyester fiber reinforcing material by impregnating a polyester fiber substrate with a first impregnating solution of a polisocyanate compound in an organic solvent, for example, trichloroethylene, perchloroethylene or toluene, and, then, by further impregnating the impregnated substrate with a second impregnating liquid containing a resorcin-formaldehyde reaction product and a rubber latex. In this process, since the polyester fibers are swollen by the organic solvent, the impregnating material can penetrate into the inside of the polyester fibers and of the polyester fiber material. Therefore, resultant polyester fiber reinforcing material can exhibit a significantly enhanced bonding property to rubber and the resultant V-belt exhibits a satisfactory resistance to abrasion. However, the use of an organic solvent results in the above-mentioned disadvantages.